ADHD is a common neurodevelopmental disorders that has been the object of intense study. A genetic basis has been firmly established through family, twin, linkage and candidate gene studies. Several strong candidate genes have emerged. Despite these advances, there remains much debate regarding the etiology of ADHD and the most appropriate ways to define the disorder and its subtypes. Part of the difficulty is due to the frequent comorbidity with several other psychiatric disorders, themselves with known heritabilities; and to the limited resolution of previous studies to characterize ADHD subtypes based on longitudinal assessments has been limited. In this project, we will thus conduct, first, exploratory data mining and, second, confirmatory analyses in a nationwide longitudinal insurance claims database spanning 2001 to present and including well over 1 million patients with a diagnosis of ADHD. The overarching aim is to inform the design and interpretation of molecular genetic studies of ADHD, especially as relates to cross-disorder analyses.